1. Field of the Invention
The present invention relates to a positioning apparatus and a positioning apparatus control method utilizing the GNSS (Global Navigation Satellite System) satellite. More particularly, the present invention relates to a GPS utilizing positioning apparatus for finding position and speed by GPS (Global Positioning System) satellites that orbit around the earth.
2. Description of Related Art
Positioning apparatuses utilizing the global navigation satellite system (GNSS) that utilizes satellites such as the GPS system run by the United States of America or GLONASS (Global Navigation Satellite System) run by Russia conventionally, calculate absolute position on the earth by receiving radio waves from a plurality of GNSS satellites at the same time and obtaining navigation messages from the GNSS satellites (orbit information and time information).
A positioning apparatus utilizing satellites usually receives four or more signals from a satellite at the same time, tracks the spreading codes by capturing the carriers, carries out spectrum despreading processing and demodulates navigation data from the satellite signals. Further, the positioning apparatus calculates the time the satellites transmitted the signals, using the navigation data and the like, finds the pseudo range of each satellite (the time the satellite signal takes to reach a positioning apparatus) and determines position of the positioning apparatus based on the determined pseudo range.
Currently, GPS receivers are widely used in various fields, such as, car navigation systems, mobile phones, airplane control and measurement of diastrophism. However, accompanying the expansion of applicable fields, it becomes more difficult for the GPS alone to satisfy required performance of the accuracy and reliability in positioning. As part of the GPS development policy, the United States of America running the GPS plans to improve the accuracy and reliability in positioning by introducing new public signals to different frequency bands in addition to the L1 band which is the existing public frequency band.
Navigation data of 50 bps showing time and position is transmitted to the ground on a signal of the existing L1 band subjected to BPSK modulation by the spreading code referred to as “C/A code (Coarse/Acquisition Code)” of 1 msec period at the carrier frequency of 1.575 GHz, the coding rate of 1.023 Mbps and the code length of 1023 chips. Two bands, the L2 band and L5 band, are planned as the new frequency bands. In the L2 band, navigation data of 25 bps is subjected to time division BPSK modulation by two spreading codes of L2CM code with code length of 10230 chips and L2CL code with the code length of 767250 chips at the carrier frequency of 1.227 GHz and the code rating of 1.023 Mbps. Therefore, the periods of codes are 20 msec and 1.5 sec, respectively. Further, in the L5 band, navigation data is subjected to QPSK modulation by the spreading code of I5/Q5 code with the code length of 10230 chips and 1 msec period at the carrier frequency of 1.176 MHz and the coding rate of 10.23 Mbps.
Patent Document 1 (Japanese Patent Application Laid-Open No. 2006-258436) discloses a satellite navigation apparatus that, in a positioning apparatus utilizing L1C/A code, L2CM code, L2CL code, L5I5 code and L5Q5 code transmitted from GPS satellites, when search is repeated upon asynchrony, carries out frequency scanning in parallel to code scanning with respect to a plurality of satellite signals, which are subjected to spectrum spreading according to different modulation codes and which are transmitted from one satellite, and captures again the carrier frequency and code phase based on the level of an added correlation value obtained by adding correlation values at a correlation detecting section.
Further, Europe plans the Galileo system for constructing a satellite positioning system using the same number of satellites as in the GPS. A plurality of frequency signals are prepared for this Galileo system, and, if a plurality of these signals can be utilized, it is possible to realize performance more than conventional one (a wider range of a receiving area and improvement of the accuracy in positioning a positioning apparatus).
However, a conventional positioning apparatus as described above increases available signals, and, so, requires signal processing circuits for the L2 band and the L5 band as the same number of channels as channels held in a current signal processing circuit for the L1 band and so there is a problem that power consumption increases accompanying the increase in the circuit scale.
Further, a plurality of signals transmitted by satellites are transmitted with different transmission power, and, compared to the L5 band utilized by the GPS, transmission power is about −2 dB in the L1 band and about −6 dB in the L2 band, and so there is a problem that it is difficult for a positioning apparatus to receive signals with little transmission power.